Aircraft components, and particularly structural members, are composed of multi-ply layers including, for example, graphite epoxy composites, titanium, aluminum, steel and combinations thereof. In the process of fastening one component or work piece of the aircraft to another, fasteners are utilized such as, for example, rivets. Rivet fasteners require the drilling of a pilot hole prior to insertion and swaging of the rivet. In prior aircraft components made of a single material, the drilling of pilot holes was relatively simple in that the drill was operated at a continuous feed rate, translational velocity, and continuous speed, rotational velocity, for the particular material. These parameters were optimized for the particular material, and the control system for these parameters could be changed when the material changed. However, the drilling of multi-ply composite material at a constant translational and rotational velocity is inefficient as well as damaging to drill bits.
In a drilling operation of a composite multi-ply work piece, it is desirable to continuously vary the translation and rotational velocities of the drill bit depending upon a material being drilled. For example, if aluminum or graphite epoxy material is being drilled, it is desirable to operate the drill at a fast rate to minimize the time required in the drilling operation. In contrast, however, if titanium or steel were drilled at a fast rate, drill bit wear would be costly. Continuous drilling through a multi-ply work piece including aluminum and titanium, for example, therefore requires separate drilling parameters for optimization. Therefore, it is necessary to instantaneously change the drilling parameters as the drill bit passes through the multi-ply work piece depending upon the configuration of the plys in the work piece. A need therefore exists for a control system for a machine tool to drill at translational and rotational velocities that are optimized to the material which comprises the workpiece.
In drilling through multi-ply work pieces, if there is a desire to change drilling parameters during the drilling operation, it is essential to make such changes so that they correspond with the material changes in the multi-ply work piece. For example, if a first layer of the work piece is titanium extending for a distance of one inch and the second layer is aluminum extending for a distance of one inch, halfway through the multi-ply layer, it will be necessary to increase the translational and rotational velocity of the drill for maximum drilling efficiency. The system must therefore have the capacity to know exactly where the material change takes place in order to effectuate the parameter changes. In order to accomplish this requirement, a need exists for a system for controlling the positioning of the drill bit prior to the drilling operation to ensure that a control system accurately monitors the position of the drill bit during the drilling operation. Since the drill bit may vary in length from operation to operation, it is necessary that the control system compensate for variations in the drill bit length prior to each drilling operation. The requirement that the drill bit tip be disposed a known distance from the work piece is also essential to ensure that the drill is moving at a predetermined translational and rotational velocity prior to contacting and entering the work piece. A knowledge of the distance between the drill bit tip and work piece is therefore critical.
A fastening operation may involve other steps in addition to drilling a pilot hole. For example, it may be necessary to insert sealant in the pilot hole prior to insertion of a rivet. The pilot hole may also require countersinking depending upon a desired application. It is therefore desired for efficient fastening that a machine tool system provide for multiple operation at a single work station including drilling, ream/countersink, insertion of sealant, insertion of rivet and swaging. A need is thus arisen for a machine tool system in which multiple functions are performed at a single work station or increasing the throughput operation of fastening components.